To turn off the friendly error message option in the web browser:

• In Internet Explorer:
  1. On the Tools menu, click Internet Options.
  2. On the Advanced tab, under the Browsing section, click to clear the Show friendly HTTP error messages check box, and then click OK.
  3. Close the browser.

• For Mozilla Firefox:
  
  1. Open up a new tab (CTRL+T) or window (CTRL+N).
  2. Type in the addressbar: about:config.
  3. In the filterbar type: error and press Enter.
  4. Now set the value of the browser.xul.error_pages.enabled to true. Double clicking will do.
  5. Restart Firefox.

Any web developer should have this feature turned off by default.

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